<!DOCTYPE html>
<html>
  <head>
    <title>mfrac linethickness</title>
    <style type="text/css">
      math {
        font-size: 10px;
      }
      @font-face {
        font-family: fraction-1;
        src: url(../fonts/math/fraction-1.otf);
      }
    </style>
  </head>
  <body>
    <math style="font-family: fraction-1;">
      <mfrac>
        <mspace height="1em" width="1em" mathbackground="red"/>
        <mspace height="1em" width="1em" mathbackground="red"/>
      </mfrac>
    </math>
      <math displaystyle="true" style="font-family: fraction-1;">
        <mfrac>
          <mspace height="1em" width="1em" mathbackground="red"/>
          <mspace height="1em" width="1em" mathbackground="red"/>
        </mfrac>
      </math>
  </body>
</html>
